As shown in FIG. 17, as means for coupling a rotary shaft 4 to a sheet metal-made rotary member 1 having a boss 2 and a peripheral portion 3 on the outer periphery thereof, such as a pulley, often used are spline coupling means for fitting a shaft spline disposed on the rotary shaft 4 to a boss spline 5 disposed in the boss 2 of the sheet metal-made rotary member 1, and key coupling means belonging to sunk keys such as a knock key or a stud key which is not shown.
When the boss spline 5 or a boss key way is to be formed in the boss 2 of the sheet metal-made rotary member 1 in which such coupling means is used, conventionally, a forming method based on a cutting process is usually employed. In the case where the boss spline 5 is to be formed by a cutting process, however, the effective thickness t1 of the boss 2 is made thinner than the thickness T of a sheet metal blank in accordance with the depth D of the boss spline 5 as shown in FIG. 18, whereby the strength of the boss 2 is correspondingly reduced. Therefore, the thickness T of the sheet metal blank must be thickened more than necessary in order to ensure the strength of the boss 2. This causes the weight of the sheet metal-made rotary member 1 to be increased, thereby blocking the reduction of the weight. As a result, the cost is raised in correspondence with the increase of the weight. The cutting process speed inevitably has a technical limitation, and hence the boss spline 5 cannot be formed in a short time. Therefore, the cost reduction due to improvement of the production efficiency cannot be expected. Moreover, there are drawbacks such as that cutting chips produced in the cutting process adversely affect the working environment. Such drawbacks are similarly caused also in the case where the boss key way is formed by the cutting process.